Hold on to those weights, Mrs. Obama!

NO DOUBT you have already heard that first lady Michelle Obama turned 50 last month - but did you hear that she was going to change her workout and do less weight training and cardio and do more yoga?

"I'm seeing myself shift from weight-bearing stuff . . . and the heavy cardio and running, to things like yoga that will keep me flexible," she told People magazine in an interview last month.

Hold up, Mrs. Obama: While yoga offers a variety of complements, including improving flexibility, yoga should not be seen as a replacement for strength training. Ideally, you want to have a well-rounded fitness routine that includes flexibility, cardiovascular and strength training.

Especially at age 50, this is the time to really invest in strength training, because most women have neglected strength training during their primary muscle-building years, which leaves many women more vulnerable to bone loss (osteoporosis).

For sure, some yoga positions will truly test your muscular strength, especially some of the arm balances and inversions, but most women are not conditioned enough to perform these gymnastic-type poses.

So, Mrs. Obama, here are 10 reasons why you, and other women 50 and older, shouldn't scrimp on strength training:

* Muscle loss: Women who do not strength train begin losing muscle as early as their mid-20s, and by the age of 70 most women have lost about 55 percent of their muscle, along with their strength and shape.

* Pump the iron: Most women, especially those 50 and older, need to build muscle, and strength training is the most practical and expedient approach to achieving that goal. Ideally, a strength-training program should incorporate progressively heavier weights to strengthen the muscular skeletal system to ward off bone loss.

* It's about strength, not bodybuilding: Not to be confused with bodybuilding, which is technically different, strength training is all about progressive resistance, which allows your muscles to adapt and gradually grow stronger over time. Bodybuilding, on the other hand, is about 80 percent diet and 20 percent exercise (weight training and cardiovascular training).

* Yoga, Pilates, running and bodybuilding: Yoga, Pilates, running and bodybuilding are not strength training. While all of the above are great adjuncts, there's no replacing pure strength training. Ladies, if you want to be strong, especially as you age, strength training is the way to go.

* Ask your doctor: Most doctors recommend weight training as the No. 1 preventive measure against bone loss. Ask your doctor how weight training can help protect your bones and prevent osteoporosis, build new bone and prevent fractures.

* Keep it challenging: Effective strength training means raising the bar, ever so slightly. With progressive resistance, as your muscles adapt and get stronger, you'll need to add a little more resistance to increase the challenge.

* You'll get leaner: Contrary to popular thought, strength training does not cause women to become larger and heavier. In fact, strength training will improve your muscle tone and improve your metabolic rate. Women do not and cannot naturally produce testosterone like men. Therefore, it's impossible for women to naturally put on Herculean-size muscle. But, muscle is "thermogenic," which means that it burns fat, and when you increase your muscle mass you'll increase your body's resting metabolic rate, and, therefore, burn more calories at rest.

* Skinny does not equal strong or fit: Skinny does not mean fit, strong or healthy. Despite what is promoted in fashion and beauty magazines, women come in various sizes and shapes, and strength should be the aspirational goal, not thinness.

* Strength training won't make you stiff: Performing strength-training exercises through the full range of motion will increase, not decrease, your flexibility. Be sure to perform the exercises correctly and you'll get a twofer out of the deal.

* Strength train and eat whatever you want: Fat chance. It's a very rare lady, indeed, who can eat whatever she wants and not gain weight. Even the statuesque Mrs. Obama, who stands at about 5'11" - 7 inches taller than the average American woman - can't afford to do that.